Motor having a rotor with interior split-permanent-magnet

ABSTRACT

A motor includes a rotor with interior permanent magnets and a stator with teeth wound by concentrated windings. Each permanent magnet is split along a plane oriented towards the stator, and an electrically insulating section is set between the split magnet pieces. This structure allows each permanent magnet to be electrically split, thereby restraining the production of an eddy current. As a result, heat-production is dampened thereby preventing heat demagnetization of the permanent magnets.

FIELD OF THE INVENTION

[0001] This application is a divisional of U.S. application Ser. No.09/471,375, filed Dec. 23, 1999.

[0002] The present invention relates generally to a motor having a rotorwith interior permanent magnets, more particularly it relates to a motorwith interior split-permanent-magnets, such that it restrainseddy-currents from occurring and prevents demagnetization of themagnets.

BACKGROUND OF THE INVENTION

[0003]FIG. 11 illustrates a rotor with interior permanent magnets of aconventional motor. The motor has rotor 310 in which permanent magnets312 are embedded, and rotor 310 is disposed in a stator (not shown) withconcentrated wounds, so that the motor can be driven by not only magnettorque but also reluctance torque. This rotor is hereinafter referred toas a “rotor with interior permanent magnets”.

[0004] However this conventional motor has the following problems:

[0005] Compared with a motor with a distributed-wound stator, a motorwith a concentrated-wound stator subjects itself to greater changes ofmagnetic flux interlinked with rotor 310 when the motor rotates. As aresult, a large-eddy-current occurs in magnets 312 embedded in therotor, and thus the motor with a concentrated-wound stator is vulnerableto irreversible demagnetization of the magnets. Meanwhile, thedistributed-wound stator is structured in the following way: A slot isformed between two stator-teeth, and a plurality of teeth thus form aplurality of slots. Wounds striding over at least one slot are provided,and part of a wound of a phase exists between pitches of another phasewound. The concentrated-wound stator, on the other hand, is structuredby providing a wound of one phase to one stator tooth respectively.

[0006] The reason why the motor having the concentrated-wound stator isvulnerable to demagnetization is detailed hereinafter.

[0007] It is well known that eddy current lost “W_(c)” is proportionateto a square of maximum operable magnetic-flux density “B_(m)”, and thisrelation can be expressed in the following equation.

W _(c) P _(t) /t={⅙ρ)}π² f ² B _(m) ² t ² [W/m ³]

[0008] wherein

[0009] P_(t)=power consumption

[0010] t=plate width interlinking with the magnetic flux

[0011] ρ=resisting value proper to the permanent magnet

[0012] f=exciting frequency

[0013] Since the motor having the concentrated-wound stator is subjectedto greater changes in magnetic flux running through the rotor, themaximum operable magnetic-flux density “B_(m)” in the above equationbecomes greater and thus eddy-current loss “W_(c)” grows larger.

[0014] If a motor has the concentrated-wound stator, and yet, thepermanent magnets are struck onto an outer wall of the rotor, thechanges in magnetic-flux-density is not so large that thedemagnetization of the magnets due to the eddy-current loss isnegligible. In the motor having the concentrated-wound stator and arotor in which the permanent magnets are embedded, the space between themagnet and the outer circumference of rotor core 314 forms a path forthe magnetic-flux to flow. The density of magnetic-flux from the statorchanges depending on the position of stator teeth with regard to themagnets, so that magnitude of changes in the magnetic-flux-density atthe path is increased. As a result, eddy-current occurs in the magnets312 embedded in rotor 310, thereby heating the magnet to produceirreversible magnetization of the magnet.

SUMMARY OF THE INVENTION

[0015] The present invention addresses the problems discussed above andaims to provide a motor having a rotor with interior-permanent-magnets.This rotor produces less eddy-current and can prevent demagnetization ofthe permanent magnets embedded in the rotor.

[0016] The motor of the present invention comprises the followingelements:

[0017] a rotor in which permanent magnets are embedded, and

[0018] a stator of which teeth are wound by wounds in a concentratedmanner.

[0019] The permanent magnets are split into magnet pieces, andinsulating sections are inserted into respective gaps between respectivemagnet pieces. This structure splits the magnets electrically, therebyrestraining the eddy-current from occurring and then suppressing thedemagnetization of the magnets embedded in the rotor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a cross-sectional view illustrating a motor, having arotor with interior permanent magnets, in accordance with a firstexemplary embodiment of the present invention.

[0021]FIG. 2 is a perspective view of the permanent magnets to beembedded into the rotor of the motor shown in FIG. 1.

[0022]FIG. 3 is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with a second exemplary embodimentof the present invention.

[0023]FIG. 4 is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with a third exemplary embodimentof the present invention.

[0024]FIG. 5 is a cross-sectional view illustrating a rotor of a motor,in which “I” shaped permanent magnets are embedded, in accordance with afourth exemplary embodiment of the present invention.

[0025]FIG. 6 is a cross-sectional view illustrating a rotor of a motor,in which permanent magnets are embedded, in accordance with a fifthexemplary embodiment.

[0026]FIG. 7A is a perspective view of permanent magnets to be embeddedinto the rotor of the motor in accordance with the fifth exemplaryembodiment.

[0027]FIG. 7B is a front view of the permanent magnets shown in FIG. 7A.

[0028]FIG. 8A is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with a sixth exemplary embodiment.

[0029]FIG. 8B is a front view of the permanent magnets shown in FIG. 8A.

[0030]FIG. 9 is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with a seventh exemplaryembodiment.

[0031]FIG. 10 is a block diagram of an electric vehicle in which themotor of the present invention is mounted.

[0032]FIG. 11 is a cross-sectional view illustrating a conventionalmotor having a rotor with interior permanent magnets.

DETAILED DESCRIPTION OF THE INVENTION

[0033] Exemplary embodiments of the present invention are demonstratedhereinafter with reference to the accompanying drawings.

[0034] (Exemplary Embodiment 1)

[0035]FIG. 1 is a cross sectional view illustrating a motor, having arotor with interior permanent magnets, in accordance with the firstexemplary embodiment of the present invention, and FIG. 2 is aperspective view of the permanent magnets to be embedded into the rotorof the same embodiment.

[0036] In FIG. 1, motor 10 includes a rotor 14 with interior permanentmagnets 12, and stator 15 facing the rotor 14 via an annular space.Respective teeth 17 of stator 15 are wound by wounds 18 in aconcentrated manner, i.e. concentrated wounds are provided on respectiveteeth.

[0037] Rotor 14 comprises the following elements:

[0038] a rotor core laminated with a plurality of steel plates;

[0039] permanent magnets 12 embedded into slots axially provided; and

[0040] a rotating shaft 16 extending through a center of the rotor core.Thus, the rotating shaft 16 provides the rotor 14 with an axis ofrotation.

[0041] Respective magnets 12 have a shape protruding toward the centerof the rotor core. As such, the magnets are embedded in the rotor sothat rotor 4 can produce respective directions for magnetic flux to flowwith ease and with difficulty. An inductance ratio in respectivedirections can be thus obtained, and it is called a salient pole rate.

[0042] A rotor polarity is formed between magnets 12 and an outer wallof the rotor core which magnets 12 face. The magnetic-flux from apermanent magnet flows with ease through the section covering the rotorpolarity, and this flowing direction is called “d axis”. On the otherhand, the magnetic-flux flows with difficulty through a section coveringa boundary between two adjacent magnets, and this flowing direction iscalled “q axis”.

[0043] Stator 15 is formed by linking twelve stator-blocks 19 to eachother in an annular shape. Each stator block 19 comprises teeth 17 woundby wounds 18 in the concentrated manner, and the blocks are welded toform a ring. In the case of a three-phase and eight-pole motor, forinstance, wounds are provided on a first four teeth, and these teeth arecoupled with each other thereby forming phase “U”. In the same manner,the wounds provided on the second four teeth on the right side of therespective first four teeth discussed above are coupled with each otherthereby forming phase “V”. Further, the wounds provided on the thirdfour teeth on the left side of the first four teeth are coupled witheach other thereby forming phase “W”. Stator 15 thus forms three-phasewith concentrated wounding.

[0044] In motor 10 constructed above, the magnetic flux generated bymagnet 12, i.e. the magnetic flux produced by the rotor-magnetic-poles,travels to teeth 17 of the stator via the annular space therebycontributing to the torque production. This motor has thesalient-pole-rate and controls the current-phases to be optimal bycurrent, thereby driving itself not only by the magnet torque but alsoby the reluctance torque.

[0045] One of the features of the present invention is a method ofembedding the permanent magnets into the rotor. Magnets 12 to beembedded into rotor 14 in the first exemplary embodiment are detailedhereinafter.

[0046] As shown in FIG. 2, each magnet 12 is split into two magnetpieces 13 in the axial direction of rotor 14. Each two magnet pieces 13are embedded into one single hole provided in rotor 14, thereby formingeach magnet 12. Epoxy resin of an electrically insulating type, used asa coating material, is applied to the overall surface of each magnetpiece 13. If magnet pieces 13 are stacked-up, each piece is electricallyinsulated and they can form an independent circuit. A space betweenrespective stacked-up magnet pieces 13 is not less than 0.03 mm,corresponding to the thickness of coating material applied to the magnetpieces.

[0047] The two magnet pieces 13 are embedded adjacently with each otherinto the hole of the rotor core so that magnet 12 is split into twosections facing stator 15. Respective magnet pieces 13 are arranged inthe following way:

[0048] Respective magnetic-fluxes generated from two magnet piecesembedded in one hole flow in the same direction with regard to the outerwall of the rotor to which these two magnet pieces face. Another pair ofmagnet pieces embedded in a hole adjacent to the hole discussed abovegenerate the magnetic flux in the direction reversed to the direction ofthe magnetic flux discussed above. For instance, two magnetic piecesembedded in one hole face the outer wall of the rotor with poles “N”,then another pair of magnet pieces embedded in the hole adjacent to thishole should face the outer wall with poles “S”.

[0049] The space between the two magnet pieces is not necessarily filledwith resin, and it can be filled with anyelectrically-insulating-material, or can include an air-gap.

[0050] Magnet 12 is split by a plane facing toward stator 15, therebyreducing the eddy current produced in magnet 12. The plane extends fromthe rotor center toward the stator. This is because of the followingreason:

[0051] Since teeth 17 are wound by concentrated wounds 18, stator 15receives greater changes in the density of magnetic-flux supplied fromteeth 17. The maximum operable magnetic-flux-density B_(m) expressed inthe equation discussed previously thus grows greater. This change in themagnetic-flux density produces the eddy current in each magnet 12. Inthis first exemplary embodiment, each magnet 12 embedded in rotor 14 issplit into two magnet pieces 13, and epoxy resin, which is non-magneticmaterial, is put between these two pieces, thereby dividing magnet 12not only physically but also electrically. As a result, the productionof an eddy current is restrained by narrowing the width “t” of a plateinterlinking with the magnetic flux in the equation discussedpreviously.

[0052] (Exemplary Embodiment 2)

[0053]FIG. 3 is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with the second exemplaryembodiment of the present invention. This second embodiment differs fromthe first one in the way of splitting the magnet; and otherwise remainsthe same.

[0054] In the first embodiment, the magnet is split into two pieces inthe axial direction, however magnet 22 in this second embodiment issplit into five pieces in the axial direction, and this produces thesame advantage as produced in the first embodiment.

[0055] (Exemplary Embodiment 3)

[0056]FIG. 4 is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with the third exemplaryembodiment of the present invention, This third embodiment differs fromthe first one in the way of splitting the magnet, and otherwise remainsthe same.

[0057] In the first embodiment, the magnet is split into two pieces inthe axial direction, however magnet 32 in this third embodiment is splitinto three pieces in a vertical direction with regard to the axialdirection, and this produces the same advantage as produced in the firstembodiment.

[0058] The first, second and third embodiments prove that the magnetssplit into pieces along planes facing the stator can restrain theproduction of eddy currents.

[0059] (Exemplary Embodiment 4)

[0060]FIG. 5 is a cross section illustrating a rotor of a motor, inwhich “I” shaped permanent magnets are embedded, in accordance with thefourth exemplary embodiment of the present invention. This fourthembodiment differs from the previous embodiments 1-3 in the shape ofmagnet. In the previous embodiments, the magnet is in a “V” shape,however, magnet 42 in the fourth embodiment is shaped like the letter“I”.

[0061] In FIG. 5, each magnet 42 formed by two magnet pieces aligned inan “I” shape is inserted into each hole provided in rotor 44.Electrically insulating material is put between the two pieces, or anair gap can be used to electrically insulate the two pieces. The fourthembodiment can produce the same advantage as produced in the firstembodiment.

[0062] Regarding the shape of the magnet, the embodiments 1-3 employ a“V” shape, and this fourth embodiment employs an “I” shape, however, theshape can be an are being bowed toward the rotor center.

[0063] (Exemplary Embodiment 5)

[0064]FIG. 6 is a cross sectional view illustrating a rotor of a motor,in which permanent magnets are embedded, in accordance with the fifthexemplary embodiment. FIG. 7A is a perspective view of the permanentmagnets to be embedded into the rotor of the motor in accordance withthe fifth exemplary embodiment, and FIG. 7B is a front view of thepermanent magnets shown in FIG. 7A.

[0065] In FIG. 6, permanent magnets 52 are embedded in rotor 54, androtary shaft 56 extends through the rotor center. This motor has astator (not shown) disposed around rotor 54 via an annular space.

[0066] Magnet 52 is formed by laminating a plurality ofrare-earth-sintered- magnet pieces. Air gaps 58 are provided betweenrespective magnetic pieces. Magnet 52 is bowed toward the rotor center.

[0067] Magnet 52 is further detailed with reference to FIGS. 7A and 7B.

[0068] Magnet 52 comprises 52 comprises a rare-earth-sintered magnet. Ingeneral, the rare-earth-sintered magnet is coated on its surface inorder to avoid corrosion. Magnet 52 is formed by laminating six piecesof this rare-earth-sintered magnet. Two or more than two protrusions areprovided on the respective faces laminated so that air gaps 58, asinsulating layers, are provided for each magnet piece. The total area ofthe protrusions formed on each magnet piece should be smaller than thearea of the face laminated, e.g. not more than 10% of the facelaminated. The number of magnet pieces is not limited to six but otherplural numbers are acceptable as far as they can provide air gapsbetween each magnet piece.

[0069] As such, since magnet 52 has insulating layers (air gaps) betweenrespective magnet pieces making up magnet 52, it is difficult forcurrent to run through magnet 52. As a result, the production of an eddycurrent is restrained. Meanwhile, magnet 52 employs a conductive coatingmaterial to avoid corrosion, however, the material can be an insulatingone, or further, respective air gaps can be filled with insulating resinthereby enhancing the strength of magnet 52. The protrusions formed oneach magnet piece can be made from another material and disposed on eachmagnet piece. Electrically insulating material among others for formingthe protrusions can produce the advantage distinctly.

[0070] (Exemplary Embodiment 6)

[0071]FIG. 8A is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with the sixth exemplaryembodiment, and FIG. 8B is a front view of the permanent magnets shownin FIG. 8A.

[0072] This sixth embodiment differs from the fifth one in the way ofsplitting the magnet, and otherwise remains the same.

[0073] In the fifth embodiment, the magnet is split into six pieces inthe axial direction, however, magnet 62 in this sixth embodiment issplit into three pieces in a vertical direction with regard to the axialdirection. The sixth embodiment can produce the same advantage asproduced in the fifth one.

[0074] (Exemplary Embodiment 7)

[0075]FIG. 9 is a perspective view of permanent magnets to be embeddedinto a rotor of a motor in accordance with the seventh exemplaryembodiment of the present invention.

[0076] This seventh embodiment differs from the fifth one in the way ofsplitting the magnet, and otherwise remains the same.

[0077] In the fifth embodiment, the magnet is split into six pieces inthe axial direction, however, magnet 72 in this seventh embodiment issplit into three pieces in a rotating direction, and a center piece ofthe three pieces is further split into five pieces in the axialdirection. The seventh embodiment can produce the same advantage asproduced in the fifth one.

[0078] When rare-earth-sintered magnets are used as interior permanentmagnets in the rotor, splitting the magnet affects the advantagedistinctly because a rare-earth-sintered magnet has less electricalresistance and, it is easier for current to run therethrough as comparedto a ferrite magnet. (The specific resistance of the ferrite magnet isnot less than 10−⁴Ω·m, and that of the rare-earth-sintered magnet is notless than 10−⁶Ω·m.). In other words, when the same magnitude of changein the magnetic-flux-density is applied from outside to the magnet, therare-earth-sintered magnet allows the eddy current to run through morethan 100 times in volume than the ferrite magnet does. Thus the split ofsuch a magnet effectively restrains the production of an eddy current.

[0079] A driving control of the motor is demonstrated hereinafter, whichmotor includes the rotor with the interior magnets of the presentinvention.

[0080] A motor with a stator wound by concentrated wounds producesgreater changes in the magnetic-flux-density when the motor is drivenunder a magnetic-field control. In the motor having a rotor withinterior permanent magnets, the magnetic-flux runs through the spacebetween the magnets and the outer circumference of the rotor core, andthus the magnetic-flux is distributed unevenly between the rotor and thestator.

[0081] The magnetic-field control applies an inverse magnetic-field tothe motor so that the magnetic-flux produced by the magnet can becounteracted, and therefore, this control method produces greaterchanges in the magnetic-flux than does a regular control method.Further, the inverse magnetic-field narrows tolerance for irreversibledemagnetization, and this produces a possibility of demagnetization at atemperature which is a matter of little concern in a normal condition.The magnetic-field-control thus produces distinctly an advantage ofdamping the heat generated by the eddy current.

[0082] It is preferable to restrain the production of an eddy current aswell as the heat-generation from the eddy current by splitting themagnet, and this shows distinctly its effect when the motor is undermagnetic-field-control.

[0083] The motor used in the embodiments discussed above is aninner-rotor type, i.e. a rotor is disposed inside a stator, however, anouter-rotor type, i.e. a rotor is disposed outside a stator, and alinear motor, i.e. a rotor moves linearly with regard to a stator,produce the same advantages.

[0084] As the exemplary embodiments discussed previously prove that themotor with interior permanent magnets of the present invention canrestrain the production of an eddy current and dampen thedemagnetization, because the magnet is electrically split and thus anarea of each magnet facing the stator becomes narrower. The motor underthe magnetic-field control can further dampen the demagnetization.

[0085] (Exemplary Embodiment 8)

[0086]FIG. 10 is a block diagram of an electric vehicle in which themotor of the present invention is mounted.

[0087] Body 80 of the electric vehicle is supported by wheels 81. Thisvehicle employs a front-wheel-drive method, so that motor 83 is directlyconnected to front-wheel-shaft 82. Motor 83 includes a stator wound byconcentrated wounds and having interior permanent magnets as describedin the exemplary embodiments previously discussed. Controller 84controls the driving torque of motor 83, and battery 85 powerscontroller 84 and further powers motor 83. Motor 83 is thus driven,which then rotates wheels 81.

[0088] In this eighth embodiment, the motor is employed to drive thewheels of the electric vehicle. The motor can be employed also to drivewheels of an electric locomotive.

What is claimed is:
 1. An electric vehicle comprising a motor which isto drive wheels of said vehicle, said motor comprising: a rotor havingan axis of rotation, and also having a first interior permanent magnetincluding at least two magnet pieces separated from one another in adirection of the axis of rotation by an electrical insulator; and astator having teeth would by concentrated windings.
 2. The electricvehicle according to claim 1, wherein said electrical insulatorcomprises epoxy resin.
 3. The electric vehicle according to claim 1,wherein said motor further-comprises at least one other interiorpermanent magnet circumferentially spaced from said first interiorpermanent magnet and including at least two magnet pieces separated fromone another by an electrical insulator, with each of said interiorpermanent magnets having an N pole and an S pole, wherein said firstinterior permanent magnet has its N pole facing said stator and eachcircumferentially adjacent said at least one other interior magnet hasits S pole facing said stator.
 4. The electric vehicle according toclaim 3, wherein said epoxy resin has a thickness of at least 0.03 mm.5. The electric vehicle according to claim 2, wherein said epoxy resinhas a thickness of at least 0.03 mm.
 6. The electric vehicle accordingto claim 1, wherein said electrical insulator has a thickness of atleast 0.03 mm.
 7. The electric vehicle according to claim 1, whereinsaid interior permanent magnet comprises a sintered magnet.
 8. Theelectric vehicle according to claim 7, wherein said electrical insulatorcomprises epoxy resin.
 9. The electric vehicle according to claim 8,wherein said motor further comprises at least one other comprising atleast one other interior permanent magnet circumferentially spaced fromsaid first interior permanent magnet and including at least two magnetpieces separated from one another by an electrical insulator, with eachof said interior permanent magnets having an N pole and an S pole,wherein said first interior permanent magnet has its N pole facing saidstator and each circumferentially adjacent said at least one otherinterior magnet has its S pole facing said stator.
 10. The electricvehicle according to claim 9, wherein said epoxy resin has a thicknessof at least 0.03 mm.
 11. The electric vehicle according to claim 8,wherein said epoxy resin has a thickness of at least 0.03 mm.
 12. Theelectric vehicle according to claim 1, further comprising a drivingapparatus for driving said motor, and a battery for powering said motor.13. The electric vehicle according to claim 12, wherein said electricalinsulator comprises epoxy resin.
 14. The electric vehicle according toclaim 13, wherein said motor further comprises at least one otherinterior permanent magnet circumferentially spaced from said firstinterior permanent magnet and including at least two magnet piecesseparated from one another by an electrical insulator, with each of saidinterior permanent magnets having an N pole and an S pole, wherein saidfirst interior permanent magnet has its N pole facing said stator andeach circumferentially adjacent said at least one other interior magnethas its S pole facing said stator.
 15. The electric vehicle according toclaim 14, wherein said epoxy resin has a thickness of at least 0.03 mm.16. The electric vehicle according to claim 13, wherein said epoxy resinhas a thickness of at least 0.03 mm.
 17. The electric vehicle accordingto claim 1, wherein said electrical insulator comprises an air gap.